1. Field of the Invention
This invention relates to a magnetic recording medium of an information storage device. More particularly, this invention relates to a magnetic recording medium suitable for achieving a high recording density and a manufacturing method of the magnetic recording medium.
2. Description of the Related Art
As an information-oriented society has made steady progress, the quantity of information that is exchanged everyday has kept on increasing. With this trend, requirements for higher recording density and greater memory capacity of information storage devices have no limits.
Magnetic recording media that have been put into practical application at present employ alloys using Co as the principal component such as Coxe2x80x94Crxe2x80x94Ptxe2x80x94B and Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta as a magnetic film. The Co alloys assume a hexagonal closed packed structure (hcp structure) with its easy axis extending in a c-axis direction. Therefore, crystallographic orientation in which the c-axis of the Co alloy assumes an in-plane direction, that is, a (11.0) orientation, is desirable as a longitudinal magnetic recording medium that executes recording by reversing magnetization inside the plane of the magnetic film. Because the (11.0) orientation is unstable, however, this orientation is not generally achieved even when the Co alloy is directly formed on a substrate.
Therefore, means has been taken that forms a (100)xe2x80x94oriented Cr underlying film before a Co alloy magnetic film is formed, by utilizing the property of the Cr (100) plane assuming a body-centered cubic structure (bcc structure) in that it has high matching with the Co (11.0) plane, and then epitaxially grows a Co alloy magnetic film on the Cr underlying film so that the c-axis of the Co alloy magnetic film faces in the in-plane direction.
To further improve crystal lattice matching in an interface between the Co alloy magnetic film and the Cr underlying film, means that adds a second element to Cr and increases the lattice gap of the Cr underlying film has been employed.
This means can further increase the (11.0) orientation of the Co alloy magnetic film and its coercivity. As examples of such technologies, JP-A-62-257618 (U.S. Pat. No. 4,652,499) and JP-A-63-197018 both disclose adding V, Ti and the like.
JP-A-11-306532 describes that when a NiTa film or a NiNb film is formed below the underlying film, the (11.0) orientation of the Co alloy magnetic film can be acquired and high recording density can be achieved. Further, JP-A-2000-503448 (officially pronounced unexamined publication) and JP-A-10-143865 describe a method that exposes a first underlying film to a predetermined oxidizing atmosphere and then forms a second underlying film as means for rendering crystal grains of a Co alloy magnetic film fine and for accomplishing the (11.0) orientation.
The factors that are necessary for achieving high recording density are high coercivity and low noise. A magnetoresitive head having extremely high reproduction sensitivity and therefore suitable for high density recording technologies has been mainly used, but when the magnetoresistive head is used, the sensitivity not only to reproduction signals from the magnetic recording medium but also to the noise becomes high. Therefore, the recording medium must have lower noise than ever.
To reduce the noise of the recording medium, it is known effective to render the crystal grains in the magnetic film fine and to uniform the crystal grain diameters. WO98/06093 re-published describes that a CrTiB alloy is effective for an underlying film for rendering the crystal grains of the magnetic film fine.
Improvement of thermal stability is another important factor in the magnetic recording medium. Thermal decay is the phenomenon that a reproduction output from the magnetic recording medium drops with passage of time. Thermal decay develops presumably because magnetization of individual crystal grains becomes thermally unstable as the crystal grains are rendered fine.
To solve the problem of this thermal decay, JP-A-2001-148110 proposes a magnetic recording medium that antiferromagnetically couples two magnetic recording layers through a nonmagnetic layer. This coupling is called xe2x80x9cAF couplingxe2x80x9d.
To accomplish a magnetic storage device having a recording density of at least 46.5 Mbit/mm2 (30 Gbit/in2), it is necessary to accomplish a magnetic recording medium having magnetic characteristics, recording/reproduction characteristics and practically sufficient thermal stability. More concretely, it is necessary to use a magnetic film that has low noise by rendering crystal grains fine, and uses AF coupling to suppress thermal decay. To further improve these characteristics of the medium, it is effective to improve crystallographic orientation inside a plane of a magnetic film formed of a Co base alloy having a hexagonal closed packed structure. When in-plane orientation of the magnetic film can be improved, magnetocrystalline anisotropy becomes strong in the in-plane direction, and coercivity of the magnetic characteristics, coercive squareness and remanence magnetization increase with the result of the improvement in recording/reproduction characteristics.
It is an object of this invention to provide a magnetic recording medium for a magnetic storage device, capable of high-density recording and reproduction of information and having less degradation of reproduction signals due to thermal decay, and a manufacturing method of the magnetic recording medium.
In the invention, the (11.0) crystallographic orientation is improved for a substrate surface of a magnetic film formed of a Co base alloy having a hexagonal closed packed structure. This (11.0) crystallographic orientation preferably has higher (11.0) diffraction intensity by X-ray diffraction so long as the size of the crystal grains remains substantially the same, and can be evaluated as having smaller dispersion of orientation and higher crystallographic orientation when a half value width of a locking curve of a (11.0) diffraction peaks is smaller.
In a magnetic recording medium including a first underlying film formed of a NiTa alloy having a nonmagnetic amorphous structure and deposited on a nonmagnetic substrate such as reinforced glass, and a second underlying film formed of an alloy containing at least Cr and Ti, a first magnetic film formed of a CoCrPt alloy, a nonmagnetic intermediate film formed of Ru and a second magnetic film formed of a CoCrPtB alloy that are serially formed over the first underlying film, the object of the invention described above can be accomplished by a magnetic recording medium wherein an interface between the first underlying film and the second underlying film is suitably oxidized. The existence of oxygen can be observed as a peak of an oxygen component in a depth profile by SIMS secondary ion mass spectroscopy.
A Pt concentration of the CoCrPt alloy film as the first magnetic film is set to about 8 at % or below so that an anisotropy field Hk of this CoCrPt alloy film is not greater than 800 kA/m and a recording head can more easily write information.
The Pt concentration is set to at least about 3 at % so as to keep lattice matching with the CrTi alloy as the second underlying film. To improve the (11.0) crystallographic orientation of the CoCrPtB alloy film as the second magnetic film, it is necessary to orient the CrTi alloy film as the second underlying film having the body-centered cubic structure to a 200orientation and to epitaxially orient the CoCrPt alloy film as the first magnetic film having the hexagonal closed packed structure on the former to the (11.0) crystallographic orientation.
A Ru film is formed as the nonmagnetic intermediate film between the first and second magnetic films. This Ru film has the hexagonal closed packed structure in the same way as the first and second magnetic films, and its film thickness is about 0.5 nm. Because Ru is formed to only several atomic layers, it hardly affects crystallographic orientation.
The closer the length of {square root over (0)}2xc3x97 a of the CrTi alloy film to the length of c of the CoCrPt alloy film, the more smoothly the first second magnetic film can be formed on the second underlying film without distortion. Consequently, the (11.0) crystallographic orientation of the first and second magnetic films can be improved. When the Ti concentration of the CrTi alloy film as the second underlying film is higher than about 15 at %, the crystallographic orientation gets deteriorated because the difference of the lattice constant from that of the CoCrPt alloy film as the second magnetic film becomes greater.
When the Ti concentration of the CrTi alloy film is smaller than about 10 at %, on the other hand, the crystal grains of the first and second magnetic films deposited over the CrTi alloy film become greater, too, so that the medium noise undesirably increases.
As the film thickness of the underlying film becomes smaller, the crystal grains become smaller, and the medium noise can be reduced during recording and reproduction. When the underlying film becomes thin, however, the (11.0) crystallographic orientation of the magnetic film is generally likely to get deteriorated. In this case, coercivity and remanence magnetization become smaller and the reproduction output drops. According to embodiments of the invention, a half value width of a locking curve of a (11.0) diffraction peak can keep at least about 8xc2x0 even when the film thickness of the second underlying film is within a small range of 5 nm to 15 nm. Since the medium noise can be reduced without lowering the reproduction output, S/N can be improved. This (11.0) diffraction peak is observed as a single peak because the peak by the first magnetic film overlaps with that of the second magnetic film.
To appropriately oxide the interface between the first and second underlying films in the magnetic recording medium according to the invention, the first underlying film is formed and then its surface is exposed to a trace amount of an oxygen gas so that a product (PO2xc2x7t) of an oxygen partial pressure and the exposure time to the atmosphere is 15 (mPaxc2x7sec) to 35 (mPaxc2x7sec) when the second underlying film is formed of CrTi, and 20 (mPaxc2x7sec) to 40 (mPaxc2x7sec) when it is formed of CrTiB, before the second underlying film is formed.
Carbon is further formed as a protective film of the second magnetic film to a thickness of 3 nm to 5 nm on the second magnetic film, and a lubricant layer of adsorptive perfluoroalkeylpolyether is disposed on a thickness of 1 nm to 2 nm. There is thus obtained a magnetic recording medium having high reliability and capable of high-density recording.
When a hydrogen-containing carbon film, a film formed of a carbide such as silicon carbide or tungsten carbide, or a mixed film of these compounds with carbon is used as the protective film, slide resistance and corrosion resistance can be improved advantageously.
When a reproduction portion of a magnetic head is constituted by a magnetoresistive head in a magnetic disk apparatus including the magnetic recording medium described above, a driving portion for driving the magnetic recording medium in a recording direction, the magnetic head having a recording portion and a reproduction portion, means for relatively moving the magnetic head to the magnetic recording medium and recording/reproduction signal processing means for executing signal input to the magnetic head and reproducing output signals from the magnetic head, a magnetic disk apparatus can be accomplished that can acquire sufficient signal intensity in high-density recording and has recording density of at least 46.5 Mbit/mm2.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.